The evolution of tape backup systems is similar to that of many computer components. At first, reel-to-reel systems (somewhat similar to old reel-to-reel audio tape recorders) were used to store data. In 1972, more than a decade before the introduction of the first IBM-PC, the 3M Company introduced the first quarter-inch tape cartridge designed for data storage. This pioneering cartridge from 3M was a dual-reel design. Somewhat more than a decade later, the first single reel tape cartridges and single reel cartridge tape drives were introduced.
Most single reel cartridges include a housing which encloses a single supply reel onto which a magnetic tape is wound. A terminator, or leader, device is generally attached to the free end of the magnetic tape. The leader device has a cross section that is larger than that of the tape, allowing it to be captured by a leader device seat within the housing whenever the tape is rewound onto the supply reel. The leader device may be in the shape of a block, a sphere, a spheroid, a rod or even a T-shaped tab. The reel has a central drive hub which is exposed by a central aperture within the housing. When the cartridge is inserted in a tape drive, a drive spindle engages the drive hub, and a priming mechanism within the drive captures a free end of the tape and either directly or indirectly threads the tape over the tape drive's read/write head and attaches the tape to a take-up reel located within the tape drive. The tape is then accessed (i.e., read and written to) in a conventional manner. After the tape has been accessed by the drive, the magnetic tape is rewound onto the cartridge's single supply reel and detached from the take-up reel so that the cartridge may be removed from the drive. This is in contrast to the dual reel tape cartridges which have both a supply reel and a take up reel incorporated therein.
Single reel tape cartridges are becoming a popular alternative to dual reel cartridges because the single reel cartridges are less expensive to manufacture and require less storage space. In fact, single reel cartridge drives are rapidly becoming the preferred design for high-capacity tape-based data backup solutions for data processing systems. Using 600 meters of 12.7 mm-wide tape wound on an 100 mm diameter reel, a single reel cartridge can store approximately 100 gigabytes of uncompressed data and approximately 200 gigabytes if compressed.
There are several disadvantages associated with the use of single reel tape cartridges. One disadvantage is increased tape drive complexity. Unlike a dual reel cartridge in which the magnetic tape is permanently attached to both reels, a single reel cartridge must be "primed", just like the reel-to-reel audio tape player/recorders that were popular forty years ago. That is, the leader device must somehow be grasped, and the attached magnetic tape threaded over the read/write head, and attached to the take-up reel. Whereas for the reel-to-reel machines, this process was accomplished by human dexterity, the process for single reel cartridges is performed automatically by a priming mechanism. The operation of a priming mechanism used for a preferred embodiment of a single reel cartridge will subsequently be briefly described. Because of the non-trivial nature of these tasks, the priming mechanism is a complex device which increases the cost of the tape drive and adds another failure mode to the tape system.
Another problem single reel drives have is that they place stress on the tape when it is rewound and the leader device is parked. This is because the cartridge reel when completely loaded with the rewound tape has considerable angular momentum. Thus, when the leader device is caught as the tape becomes fully rewound with the cartridge reel spinning, the magnetic tape may be stretched or broken by the shock loading. Consequently, most single reel tape drives currently in production dramatically reduce rewind speed as the tape approaches the fully rewound condition in order to reduce the angular momentum to safe levels which will not break or permanently stretch the tape.
Another problem related to the use of single reel tape cartridges involves the need to maintain tension on the tape after the cartridge has been removed from the tape drive mechanism. Although reel locking mechanisms have reduced the amount of slack that can be introduced, tension cannot be maintained because the locking mechanisms generally in use are not rotationally continuous, but have a plurality of discreet locking positions. If tension is not maintained, the outer layers of tape will become loose and may slip. Not only will this slippage result in a certain amount of tape feed misalignment, but contact between tape edges and the supply reel flange may cause increased tape wear and some additional friction as the tape is unwound from the supply reel.
A single reel tape cartridge 10 is depicted in FIGS. 1 and 2. The cartridge 10 includes a housing 11 which encloses a tape supply reel. A sliding door 12 covers the tape access port and leader device parking place (neither of which are shown in this view). The door 12 slides to the side in order to expose the tape access port when the single reel tape cartridge 10 is inserted in a tape drive. A driven gear (also referred to herein as a driven coupler) 21 is incorporated in the central hub of the tape supply reel (the tape reel itself is enclosed by the housing 11 and is not visible in this view). The driven gear 21 engages a driver gear (also referred to herein as a driver coupler) which is part of the tape drive (the driver gear is not shown). It will be noted that the driven gear has a central aperture 22 through which a reel locking mechanism within the cartridge 10 may be released. Although driver and driven gears are utilized for imparting rotary motion to the supply reel other types of couplers using splines or friction may be substituted for the geared type.
A reel locking mechanism 30 is depicted in a cut-away view of the single reel cartridge of FIGS. 3 and 4. The driven gear 21 is rigidly affixed to the reel 31. The backside of the driven gear 21 (i.e., the side which does not engage the driver gear) has a first locking gear 32 rigidly affixed thereto. The first locking gear 32, which has a first set of teeth 33, is centered about the reel's axis of revolution 38, and rotates with the reel 31 and driven gear 21. The first set of teeth 33 of the first locking gear 32 engage a second set of teeth 34 on a second locking gear 35 when the driven gear 21 is decoupled from the driver gear 36 of the tape drive. The second locking gear 35, which is non-rotatable and slidably mounted on a pedestal 37 rigidly affixed to the drive housing 11, is resiliently biased toward the first locking gear 32 along the rotational axis 38 of reel 31. Resilient biasing of the second locking gear may be provided by a coil spring (not shown) within the pedestal 37 which is compressed between the pedestal and the second locking gear 35. When the driver gear 36 of the tape drive engages the driven gear 21, a projection 39 on the top of the driver gear 36 passes through a center aperture in the driven gear 21, lifting the second locking gear 35 a distance at least sufficient to disengage its teeth from those of the first locking gear 32. Each tooth of the first and second sets of teeth (33 and 34, respectively) are preferably ramp shaped so that when the teeth of both locking gears are engaged, rotation of the reel 31 in an unwind direction is blocked. At least one of the locking gears must have a full complement of equally-spaced teeth. As there are a finite number of teeth on each locking gear, the reel 31 may be locked in a plurality of positions equal to the number of teeth on a locking gear having a full-complement of teeth. The angular distance between each locking position will be equal to 360 degrees divided by the total number of locking positions. For example, if each of the locking gears has a total of 60 teeth, the reel may be locked in 60 different angular positions, six degrees apart, as it is rotated. Thus, when the tape is rewound completely and still taught, that same degree of tautness will remain only if the teeth of both locking gears are perfectly aligned with one another. If the gears are not perfectly aligned, up to six degrees of slack are possible. The linear slack in the tape may be readily calculated. For example, the maximum amount of slack in a tape on a reel having an outermost loop with a diameter of 100 mm and a circumference of approximately 314 mm will be about 314/60, or about 5 mm. Although the amount of tape slack will average half that amount, or about 2 mm, the system must be able to cope with the worst-case scenario of 5 mm. Tape slack of 5 mm may cause significant tape slippage. One might reasonably assume that by increasing the number of teeth on the locking gear, tape slack could be reduced. Although this is theoretically correct, there is a limit to the number of teeth (particularly plastic teeth) that may be placed on the locking gear before the teeth are likely to strip and become useless with repeated use.
What is needed is an improved tape leader device parking mechanism which will not only reduce stresses on the magnetic tape or on a leader tape attached to the magnetic tape when the leader device is captured in its parking place under rewind conditions, but which will also maintain tension on the tape after it has been rewound into the cartridge, removed from the tape drive, subjected to handling which may be not particularly gentle, and stored for extended periods of time in varying temperature conditions.